The background and a brief summary of the invention are contained in Griffin, "Combustor Adapted to Direct Fire Kilns", Timber Processing Industry, Aug. 1980, front cover and pp 10-12 and 14.
Until recently it was the almost unexceptioned practice to dry lumber after sawing, using the burning of fossil fuel, particularly natural gas to create the necessary heated-gas stream for circulation through the kiln. Various proposals had been made and tried for direct-fired combustors using wood waste, particularly sawdust, as fuel, but incompleteness of combustion has always represented a difficult problem. Lately, as the cost of petroleum has risen, the desire to switch to sawdust as fuel for heating the lumber kiln gas stream has become irresistible. Fortunately, there have been a number of improvements made in combustors in recent years, e.g. the development of fluidized-bed combustion techniques, so that it is now reasonable to burn sawdust, or a mixed fuel largely consisting of sawdust, to produce the heated dry gas stream for kilning lumber.
One unavoidable product of the combustion of sawdust is ash: fine particulate material consisting of that which would not or did not burn completely and that which when burned remained or produced a solid. Under most operating conditions, the great bulk of the ash will fail to become entrained in the heated gas stream, and if so-entrained, will be carried through the kiln in the heated gas stream and removed in the bag house filters on the downstream side of the kiln. However, a certain amount of the particulates will silt-out within the kiln, and some of this will deposit on the lumber being kilned.
Typically, within a modern lumber kiln, a multi-course pile of lumber is subjected as a unit to the heated gas stream. In order to ensure even and thorough treatment the various courses are stacked with the aid of spacing courses--each being an open layer comprising a few sticks placed crosswise to the direction of the lengths of lumber in the immediately subjacent and superjacent courses.
Accordingly, as the stacked lumber is being kilned in a direct-fired sawdust fueled combustor, one can expect the pile of lumber exiting the kiln to bear a deposit of fine ash not just on the outer, upper or exposed lengths in the pile, but throughout the pile, potentially on every piece in each course.
A piece of apparatus has been developed and become widely used for converting the pile of kilned lumber into a single layer of uniformly oriented lengths for further processing, e.g. for planing. One type of such controlled de-stacking apparatus is known as a breakdown hoist.
At a breakdown hoist, one lateral side of the pile of kilned lumber is supported using a weir-like wall means along the full height of the pile and then the pile and wall are tilted so that all courses are supported against this wall. Next the wall is gradually lowered relative to the tilted stack so that support is serially withdrawn from first the upper course of lumber, then from the layer of spacer sticks which immediately underlaid that layer, and so on. Usually, this relative lowering is effected by raising the stack relative to the wall, so that the site where the layers become unsupported remains stationary. Immediately downstream of the site where the uppermost layer of lumber in the tilted pile becomes exposed on its downward-tilted side, there is provided a feeding deck, which may take the form of an inclined set of rails or the like leading down to a conveyor. Usually, this conveyor is the feed conveyor for a further processing station, e.g. a planer mill. A provision is generally made so that each layer of the spacer sticks as it becomes exposed falls down between the tilted stack and the feeding deck, and each layer of kilned lumber as it becomes exposed tumbles down the incline and onto the feed conveyor.
Where an ash-producing direct fired combustor has been used to kiln the lumber, this operation of destacking by tilting, sliding and tumbling is accompanied by the billowing-up of clouds of ash as it is violently shaken-free of the kilned lumber and spacer sticks. And that which remains on and around the lumber entering the planer mill acts as an abrasive on the moving and cutting parts, so that the bearings, planer knives and the like become worn and dull at an excessive rate.